Issues playing Dolby Vision files with external subs on LG C2

Valasty · Post by **Valasty** » Tue Apr 29, 2025 1:31 pm

Hello friends!

First of all, thank you very much for the support on UMS, it's amazing and I've been using it for many years!

Now the issue... we recently started getting Dolby Vision movies and series for download, and I managed to get UMS to successfully reproduce them by activating option "Remux videos with tsMuxeR when possible instead of transcoding" (saw it in another thread and it worked like a charm).
This is AWESOME news for me since my TV supports it (LG C2) and it looks AMAZING!

But here are the problems:
- The player doesn't seem to recognize built-in subtitles, the TV menu for subtitles is blanked out. This happens only with DV files, everything else works perfectly.
- If I add an external subtitle it loads normally, but for some reason they have a 0.5 delay. It's no big deal, I can easily readjust it in the menu.

Now here comes the strange part...
If I add an external subtitle I can no longer move the movie forward/backward because if I do, the colors starts changing and my CPU goes crazy! It immediately jumps from 60º to 76º and keeps increasing, and with that a lot of fan noise.
This never happens as long as I don't go forward/backward, and pausing is also fine.
This also doesn't happen without external subtitles, I freely move forward/backward and my CPU barely aknowledges it.

I don't know much about encoding so I didn't mess up with almost any options there aside from the one I mentioned in the first paragraph. I did try changing some stuff around but nothing seemed to work, so I just put them back on default.

For reference, this is the file name: Andor S02E02 Sagrona Teema 2160p DSNP WEB-DL DDP5 1 DV H 265-NTb (also tested with the DV HDR release, same problem)

Steps to reproduce:
1. Play the file without external subtitles
2. Move 11 mins forward (or any amount actually)
3. Play the file with external subtitles
4. Repeat 2 -> bug
5. You can repeat steps 1 and 2 to confirm as well, it won't have issues.

Attached is the log playing the same file doing the steps above.

BONUS ISSUE! (yeah, sorry lol)
There's also a whole different issue I've encountered only in a single file so far, so I'm not going very in depth here since it would probably be too much for this topic, but maybe it's related somehow. It's a DV HDR file (so it's not pink and green image) but this problem only happens on this specific file...
- Without external subtitles it loads as HDR, with external subtitles it loads as DV (yeah, I know lol)
- When loaded as DV the audio is totally out of sync.
- If I go backwards/forward the audio is corrected, but the image quality and the CPU goes crazy.

This is the release name for reference: The.Gorge.2025.Hybrid.2160p.WEB-DL.DV.HDR.DDP5.1.Atmos.H265-AOC

Any help would be very much appreciated!

Post by **mik_s** » Tue Apr 29, 2025 4:03 pm

I checked the logs and can see that UMS has to transcode that video for the TV to be able to play it.

This is because dolbyvision is not set as supported in MKV files in the conf file.
I thought it was strange that as it is set for MP4 but after a bit of googling it appears LG TVs cannot play dolbyvision inside MKV files.
I found a thread on here that mentions it and others like this

Because of that what UMS does is copies the video and audio streams from the MKV as otherwise they are supported and remuxes them with TsMuxeR into MPEG so the TV can play it.

When there are external subs these will be sent to the TV as a separate stream however the internal subs are not accessible so cannot be used.
There was an attempt to extract subs from videos for them to be used externally but it never worked so this idea was abandoned.
The delay in the external subs may be due to the mismatch in the sources of the video and sub files and they may not be synced.

If the video has forced internal subs for foreign/alien language parts and there is no external subtitles the only way for these to show is to burn them in to the video which means it cannot be remuxed and has to be transcoded instead. This takes a lot of processing power so that is why the CPU is working hard.

You can offload most of the processing to the GPU if it has a video encoder. Check the settings for FFmpeg transcoding

: Screenshot 2025-04-29 044347.png (7.76 KiB) Viewed 24069 times

: Screenshot 2025-04-29 044325.png (27.38 KiB) Viewed 24069 times

Transcoding has the downside of having problems with seeking on some players as the video stream is destroyed after being sent to the TV so it takes a while to restart transcoding if you seek to another point. Some players will even disable seeking altogether.

Seeking while it is being remuxed may mess up the process too causing it to fall back onto full transcode.

I don't know anything about HDR as I don't have a HDR TV but I have heard that colour change is due to a different HDR format.
I am not sure if the DV data is kept while being transcoded so this would explain the colour change.

The same thing is happening with the second video but the audio has to be transcoded due to Atmos not listed as supported.

There is not really much that can be done to improve things.
I can only suggest you avoid DV MKV files so they won't need to be transcoded or you can re-encode them into MP4. I think MKVtools allows you to do this.

I think I saw that if DV does not work it can fall back on another HDR format.
You can try by playing from the #--TRANSCODE--# folder (you will have to enable it in the settings if not showing) and chose "no transcoding" to send the video unaltered to the TV, but it may just ignore the DV data and play without.
If it does play with DV working then I can tweak the conf to add dolbyvision as supported in MKVs.

Valasty · Post by **Valasty** » Wed Apr 30, 2025 2:22 am

Thanks for the feedback, I will give those GPU transcoding a try!

I get what you're saying that there's no fix for internal subtitles, but I think you missed the main point of my issue... the DV files works perfectly fine if there is no external subtitles.
It also works fine with external subtitles if I don't move the timeline, the CPU barely moves to transcode/remux.

And the small delay on the subs I'm pretty sure the issue is not the file, this only happens on these DV files, regardless if which. They are always in sync in the PC.

Post by **mik_s** » Fri May 02, 2025 5:55 am

I had another look and see what you mean. I don't know why but it is transcoding when there are subs present.
I think this is all the relevant parts of the log

Code: Select all

Asked stream chunk: TimeRange [start=673.308, end=null] of video.mkv and engine FFmpeg Video (MPEGTS-H265-AC3)
Setting lastStartPosition from time-seeking: 673.308
Requesting time seek: 673.308 seconds
Setting last time seek to: 673.308 seconds

As a new section is being asked for the previous stream is stopped

Code: Select all

IPC pipe interrupted after writing 0 bytes, shutting down...
Stopping process 11260: ffmpeg.exe-9
Windows named pipe interrupted after writing 0 bytes, shutting down...
Windows named pipe interrupted after writing 0 bytes, shutting down...

This may be the cause if this was not handled properly.

Code: Select all

java.io.InterruptedIOException: null
	at java.base/java.io.PipedInputStream.awaitSpace(Unknown Source)
	at java.base/java.io.PipedInputStream.receive(Unknown Source)
	at java.base/java.io.PipedOutputStream.write(Unknown Source)
	at net.pms.platform.windows.WindowsNamedPipe.run(WindowsNamedPipe.java:224)
java.io.InterruptedIOException: null
	at java.base/java.io.PipedInputStream.awaitSpace(Unknown Source)
	at java.base/java.io.PipedInputStream.receive(Unknown Source)
	at java.base/java.io.PipedOutputStream.write(Unknown Source)
	at net.pms.io.PipeIPCProcess.run(PipeIPCProcess.java:81)

Which could result in a black screen

Code: Select all

Not muxing the video stream with tsMuxeR via FFmpeg because the renderer will display a blank screen, no good explanation for this yet.

I don't know why subtitles would require the audio to be transcoded too as they are streamed seperatly.

Code: Select all

Audio was not remuxed because there are subtitles and seeking involved

This appears to be exactly the same in both logs whether there are subs or not except that final entry.
Maybe that is causing the video to be fully transcoded as without subs it is just copying the streams.

I'll make an issue for this for the devs to have a better look.

Valasty · Post by **Valasty** » Fri May 02, 2025 9:17 am

Awesome, thanks!
Please let me know if you need additional logs/help/testing.

jaapie18 · Post by **jaapie18** » Mon May 05, 2025 5:51 am

with this tool xmedia recode. You can convert mkv to mp4 and the dolby vision stays intact.

Universal Media Server | Forum

Issues playing Dolby Vision files with external subs on LG C2

Issues playing Dolby Vision files with external subs on LG C2

Re: Issues playing Dolby Vision files with external subs on LG C2

Re: Issues playing Dolby Vision files with external subs on LG C2

Re: Issues playing Dolby Vision files with external subs on LG C2

Re: Issues playing Dolby Vision files with external subs on LG C2

Re: Issues playing Dolby Vision files with external subs on LG C2